First, try to verify that the leak is coming from the seal and not one of the lower bolts. The lower bolts will leak if they have split lock washers - should have solid washers.
You can unbolt the shaft coupling from the transmission flange, and then slide the shaft and coupling aft until the coupling hits the stuffing box. That might give you room to remove the transmission flange.
First piece forward of that carries the seal. Remove the piece, replace the seal, reinstall. During reinstall, remember that the lower bolts get solid washers, not split.
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